1. Field of the Invention
The present invention relates to a method of and an apparatus for forming a single-crystalline thin film on a substrate, and it relates to a method of and an apparatus for forming a single-crystalline thin film, which implement selective and efficient formation of a single-crystalline thin film, and it also relates to a beam irradiator, a beam irradiating method, and a beam reflecting device for enabling efficient formation of a single-crystalline thin film or an axially oriented polycrystalline thin film on a substrate.
2. Background of the Invention
Plasma chemical vapor deposition (plasma CVD) is a type of chemical vapor deposition process (CVD), which is adapted to bring a reaction gas into a plasma state for forming active radicals and ions and to cause a chemical reaction under active environment, thereby forming a thin film of a prescribed material on a substrate at a relatively low temperature. The plasma CVD, which can form various types of films at low temperatures, has such advantages that it is possible to form an amorphous film while preventing crystallization, to employ a non-heat-resistant substrate such as a plastic substrate, and to prevent the as-formed film from reacting with the substrate. Therefore, plasma CVD has many applications in the semiconductor industry.
It is possible to epitaxially form a single-crystalline thin film of a prescribed material on a single-crystalline substrate by carrying out the plasma CVD under a temperature facilitating crystallization.
Generally, in order to form a single-crystalline thin film of a prescribed material on a single-crystalline substrate of the same material having the same crystal orientation, it is possible to employ an epitaxial growth process. In the epitaxial growth process, however, it is impossible to form a single-crystalline thin film on a polycrystalline substrate or an amorphous substrate. Therefore, in order to form a single-crystalline thin film on a substrate having a different crystal structure such as an amorphous substrate or a polycrystalline substrate, or a substrate of a different material, an amorphous thin film or a polycrystalline thin film is temporarily formed on the substrate so that the same is thereafter converted to a single-crystalline thin film.
In general, a polycrystalline or amorphous semiconductor thin film is single-crystallized by fusion recrystallization or lateral solid phase epitaxy.
However, such a process has the following problems: In the fusion recrystallization, the substrate is extremely thermally distorted when the thin film is prepared from a material having a high melting point, to damage physical and electrical properties of the thin film as employed. Further, an electron beam or a laser beam is employed for fusing the thin film. Therefore, it is necessary to scan spots of the electron beam or the laser beam along the overall surface of the substrate, and hence a long time and a high cost are required for recrystallization.
On the other hand, the lateral solid phase epitaxy is easily influenced by a method of crystallizing the material forming the substrate, while the growth rate is disadvantageously slow in this process. In order to grow a single-crystalline thin film over a distance of about 10 .mu.m, for example, this process requires at least 10 hours. Further, it is difficult to obtain a large crystal grain since a lattice defect is caused to stop growth of the single crystal upon progress of the growth to some extent.
In each process, further, it is necessary to bring a seed crystal into contact with the polycrystalline or amorphous thin film. In addition, the single crystal is grown in a direction along the major surface of the thin film, i.e., in a lateral direction, whereby the distance of growth to the crystal is so increased that various hindrances take place during the growth of the single crystal. When the substrate is made of an amorphous material such as glass, for example, the substrate has no regularity in lattice position and this irregularity influences on growth of the single crystal to disadvantageously result in growth of a polycrystalline film having large crystal grain sizes. In addition, it is difficult to selectively form a single-crystalline thin film having a prescribed crystal orientation on an arbitrary region of the substrate, due to the lateral growth.
In order to solve the aforementioned problems of the prior art, there has been made an attempt for reducing the growth distance by utilizing vertical growth of the thin film, thereby reducing the growth time. In other words, there has been tried a method of bringing a seed crystal into contact with the overall surface of a polycrystalline or amorphous thin film for making solid phase epitaxial growth in a direction perpendicular to the major surface of the thin film, i.e., in the vertical direction. As a result, however, the seed crystal was merely partially in contact with the amorphous thin film or the like and it was impossible to form a single-crystalline thin film by the as-expected vertical solid phase epitaxial growth, since only lateral epitaxial growth was caused from the contact portion. According to this method, further, the seed crystal adhered to the as-grown single-crystalline film and it was extremely difficult to separate the former from the latter, such that the as-grown thin film was disadvantageously separated from the substrate following the seed crystal. Further, it is impossible in practice to selectively form a single-crystalline thin film having a prescribed crystal orientation on an arbitrary region of the substrate, since it is necessary to accurately arrange a seed crystal of a prescribed shape on a prescribed position.
When the substrate itself has a single-crystalline structure, it is impossible to form a single-crystalline thin film having a crystal orientation which is different from that of the substrate on the substrate by any conventional means.
This also applies to a polycrystalline thin film having single crystal axes which are regulated along the same direction between crystal grains, i.e., an axially oriented polycrystalline thin film. In other words, it is difficult to form an axially oriented polycrystalline thin film which is oriented in a desired direction on an arbitrary substrate by the prior art.